1. Technical Field:
This invention relates to a process for selectively plugging high permeability zones of a subterranean hydrocarbon-bearing formation by enhancing the delay associated with the in situ formation of a polymer gel to improve conformance and flow profiles of fluids injected after gelation.
2. Description of Related Art:
Poor vertical conformance occurs when subterranean formations have vertical juxtapositions of relatively high permeability geologic regions to relatively low permeability geologic regions. Poor areal conformance occurs when subterranean formations contain high permeability streaks and high permeability anomalies within the formation matrix, such as vertical fractures and fracture networks, which have high permeability relative to the formation matrix. Fluids generally exhibit poor flow profiles and sweep efficiencies in subterranean formations having poor vertical or areal conformance. Poor conformance is a problem where structural anomalies such as fracture networks and vertical heterogeneity are in fluid communication with a subterranean well bore across which fluids are injected or produced.
A number of attempts to remedy conformance problems exist. U.S. Pat. Nos. 3,762,476; 3,981,363; 4,018,286; and 4,039,029 to Gall or Gall et al. describe various processes where gel compositions are formed in high permeability regions of subterranean formations to reduce the permeability therein. According to U.S. Pat. No. 3,762,476, a polymer such as polyacrylamide is injected into a formation followed by a crosslinking agent. In practice, treatments such as that disclosed in U.S. Pat. No. 3,762,476 using sequentially injected gel systems have proven unsatisfactory because of the inability to achieve complete mixing of the crosslinking agent with the polymer, which results in incomplete gelation within the formation. Incomplete gelation causes a continuation of poor vertical or areal conformance, poor flow profiles and poor sweep efficiencies within the formation. This problem led to the development of delayed polymer gel techniques.
An attempt to achieve delayed polymer gelation was U.S. Pat. No. 4,706,754 to Smith which describes a process for reducing formation permeability by using polyacrylamide gels in combination with a crosslinking agent and a carboxylic acid delaying agent. The delaying agent allows for the gelation solution to thoroughly mix and be placed in the desired treatment region up to about 24 hours before gelation occurs. However, it is often desirable to delay gelation for longer than 24 hours to enable a gel to be placed at distances from the injection well that takes longer than 24 hours to achieve.
U.S. Pat. No. 4,744,418 to Sydansk discloses that at temperatures below 140.degree. F. (60.degree. C.), gelation can be delayed by decreasing the amount of polymer hydrolysis. U.S. Pat. No. 4,844,168 also to Sydansk discloses that above temperatures of about 140.degree. F. (60.degree. C.) polyacrylamide autohydrolysis increases at a rate that is proportional to the increase in temperatures. Sydansk also discloses that delayed gelation can be achieved at temperatures above about 140.degree. F. (60.degree. C.) by utilizing polyacrylamide having less than 0.1% hydrolysis. Sydansk is able to achieve delays in gelation of up to about 168 hours at about 165.degree. F. (74.degree. C.) and up to about 20 hours at temperatures of about 219.degree. F. (104.degree. C.). However it is often desirable to delay gelation for longer than 168 hours at temperatures of about 165.degree. F. (74.degree. C.) and longer than 20 hours at temperatures of about 219.degree. F. (104.degree. C.). Therefore a need exists for a gelation process wherein gelation delays can be enhanced when reservoir temperatures exceed 165.degree. F. (74.degree. C.) and exceed 219.degree. F. (104.degree. C.).
It is therefore an object of this invention to provide a process for selectively plugging high permeability zones of a subterranean hydrocarbon-bearing formation by use of an aqueous polymer gelation solution in which gelation is delayed for several hours to several weeks when formation temperatures are near 176.degree. F. (80.degree. C.). It is a further object of this invention to provide a delayed gelation solution which can be used at temperatures above about 219.degree. F. (104.degree. C.) where gelation can be delayed for several hours to about one day.